The present invention relates to atmospheric corona discharge devices, and in particular, to an improved electrode for corona discharge devices.
Atmospheric, direct current (DC), corona discharge is used to provide a unipolar ion source for a variety of electrical devices including air cleaners, in which the ions charge particulates to draw them to a collector plate, and photocopiers and laser printers, in which the ions charge a photosensitive drum.
Atmospheric corona discharge, as its name suggests, employs a discharge electrode surrounded by air. A steep electrical gradient at the discharge electrode produces a plasma of ionized atoms or molecules near the discharge electrode. Some ions escape from the plasma region to form charge carriers that migrate to a second electrode. Atmospheric corona discharge is readily distinguishable from devices that provide a stream of electrons such as field emission devices and thermionic emission devices, each of which normally operate in a near or complete vacuum.
The plasma region in which the ions are generated may convert atmospheric oxygen (O2) to ozone (O3), the latter being a reactive gas that in high concentrations can be a health concern. Ozone can be reduced by using a positive voltage at the discharge electrode. Ozone can also be reduced by limiting discharge current, but at the cost of reducing the number of ions generated, and thus reducing the effectiveness of the associated equipment. Air temperature and air velocity are not major factors in the control of ozone creation for most indoor applications.
The ionization of air by the discharge electrode is influenced by the sharpness (radius of curvature) of the discharge electrode such as increases the gradient of the electrical field about the discharge electrode. This relationship is captured in the empirically derived Peek's equation. Experimental data for different electrode radii as low as 10 micrometers also indicate a reduced ozone production for a given surface current density as the electrode radius decreases.
For these reasons, commercial corona devices have employed wire electrodes as small as one micrometer in radius. Such wires provide a small radius of curvature, reducing ozone production and discharge voltage (and thus discharge power consumption) while maintaining an acceptable ion production rate.
The ability to further decrease the wire size is limited by practical considerations of wire strength and durability in the typical operating environment of an atmospheric corona discharge device.